Aircraft have used on-board inert gas generating systems (OBIGGS) to protect against fuel tank explosions by replacing the potentially explosive fuel vapor/air mixture above the fuel in the ullage space of the tanks with nitrogen-enriched air (NEA). The NEA is generated by separating oxygen from local, ambient air and pumping the inert, oxygen impoverished NEA into the tanks.
Production of NEA typically is carried out by means of an apparatus relying on permeable membranes, or else on molecular sieves. The air separation apparatus is generally referred to as an air separation module (ASM). A selectively permeable membrane ASM typically comprises a bundle of hollow fiber membranes packaged in a cylindrical shell with an inlet and outlet at the ends of the shell, and a shell side vent port. When pressurized air enters the ASM shell and passes through the hollow fibers, oxygen is separated from the air stream due to diffusion through the fiber walls. That is, the fiber walls are more permeable to oxygen than nitrogen. The oxygen exits through the side vent port and can be recaptured, but often the oxygen is considered a waste gas that is exhausted overboard. The remaining NEA flows out of the ASM via the outlet port and is distributed to the ullage space of the fuel tank or tanks for the purpose of inerting the fuel tanks and thereby reducing flammability. The ASM operates most efficiently, in terms of permeability of oxygen over nitrogen, at an elevated temperature higher than ambient temperature. The selective permeability has a direct relationship to the purity of the NEA (the more nitrogen and less oxygen, the higher the purity).
In many if not most commercial airplane applications, pressurized air used for NEA generation will originate from either an engine bleed or from a cabin air pressure source. With an engine bleed pressure supply, compressed hot air is ducted from an engine bleed air supply line and then cooled by a heat exchanger to an optimal temperature for maximum ASM performance. This use of engine bleed air can decrease engine performance and can lead to increased fuel consumption. Accordingly, it is desirable to limit the amount of engine bleed air that is needed during the various segments of the aircraft flight profile and particularly during cruise.
In a known OBIGGS architecture, plural ASMs are provided to allow for low NEA flow from a primary ASM during the cruise phase of an aircraft flight profile and high NEA flow from both the primary and one or more additional secondary ASMs during aircraft descent. During cruise, the primary ASM receives a steady flow of pressurized air at a controlled temperature, thereby maintaining the primary ASM at a desired operating temperature for providing a high purity NEA. The secondary ASMs, however, are idle, i.e. shut off from the elevated temperature supply air, and can become thermally soaked at surrounding ambient bay temperatures considerably lower than the optimal operating temperature of the ASM. Consequently, when the pressurized heated supply air is supplied to the secondary ASMs for high NEA flow during descent, initially the secondary ASMs will not be at the desired elevated temperature and thus will produce low purity NEA.